Analog read and write paths in a solid state memory device

ABSTRACT

A memory array in a memory device is coupled to an analog I/O data interface that enables analog voltage levels to be written to the memory array. The I/O interface comprises a plurality of analog data paths that each includes a capacitor for storing charge corresponding to a target voltage to which a selected memory cell, coupled to its respective data path, is to be programmed. A plurality of comparators can be included in the I/O interface, with each such comparator coupled to a respective bit line. Such a comparator can compare a threshold voltage of a selected memory cell to its target voltage and inhibits further programming when the threshold voltage equals or exceeds the target voltage.

RELATED APPLICATION

This application is a Continuation of U.S. application Ser. No.12/098,652, titled “ANALOG READ AND WRITE PATHS IN A SOLID STATE MEMORYDEVICE” filed Apr. 7, 2008, now U.S. Pat. No. 7,768,832, issued Aug. 3,2010, which is commonly assigned and incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory andmore particularly, in one or more embodiments, to non-volatile memorydevices.

BACKGROUND

Electronic devices commonly have some type of bulk storage deviceavailable to them. A common example is a hard disk drive (HDD). HDDs arecapable of large amounts of storage at relatively low cost, with currentconsumer HDDs available with over one terabyte of capacity.

HDDs generally store data on rotating magnetic media or platters. Datais typically stored as a pattern of magnetic flux reversals on theplatters. To write data to a typical HDD, the platter is rotated at highspeed while a write head floating above the platter generates a seriesof magnetic pulses to align magnetic particles on the platter torepresent the data. To read data from a typical HDD, resistance changesare induced in a magnetoresistive read head as it floats above theplatter rotated at high speed. In practice, the resulting data signal isan analog signal whose peaks and valleys are the result of the magneticflux reversals of the data pattern. Digital signal processing techniquescalled partial response maximum likelihood (PRML) are then used tosample the analog data signal to determine the likely data patternresponsible for generating the data signal.

HDDs have certain drawbacks due to their mechanical nature. HDDs aresusceptible to damage or excessive read/write errors due to shock,vibration or strong magnetic fields. In addition, they are relativelylarge users of power in portable electronic devices.

Another example of a bulk storage device is a solid state drive (SSD).Instead of storing data on rotating media, SSDs utilize semiconductormemory devices to store their data, but include an interface and formfactor making them appear to their host system as if they are a typicalHDD. The memory devices of SSDs are typically non-volatile flash memorydevices.

Flash memory devices have developed into a popular source ofnon-volatile memory for a wide range of electronic applications. Flashmemory devices typically use a one-transistor memory cell that allowsfor high memory densities, high reliability, and low power consumption.Changes in threshold voltage of the cells, through programming of chargestorage or trapping layers or other physical phenomena, determine thedata value of each cell. Common uses for flash memory and othernon-volatile memory include personal computers, personal digitalassistants (PDAs), digital cameras, digital media players, digitalrecorders, games, appliances, vehicles, wireless devices, mobiletelephones, and removable memory modules, and the uses for non-volatilememory continue to expand.

Unlike HDDs, the operation of SSDs is generally not subject tovibration, shock or magnetic field concerns due to their solid statenature. Similarly, without moving parts, SSDs have lower powerrequirements than HDDs. However, SSDs currently have much lower storagecapacities compared to HDDs of the same form factor and a significantlyhigher cost per bit.

For the reasons stated above, and for other reasons which will becomeapparent to those skilled in the art upon reading and understanding thepresent specification, there is a need in the art for alternative bulkstorage options.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a memory device according to anembodiment of the disclosure.

FIG. 2 is a schematic of a portion of an example NAND memory array asmight be found in the memory device of FIG. 1.

FIG. 3 is a block schematic of a solid state bulk storage system inaccordance with one embodiment of the present disclosure.

FIG. 4 is a depiction of a wave form showing conceptually a data signalas might be received from the memory device by a read/write channel inaccordance with an embodiment of the disclosure.

FIG. 5 is a block schematic of an electronic system in accordance withan embodiment of the disclosure.

FIG. 6 is a block diagram of one embodiment of a memory device, inaccordance with the bulk storage system of FIG. 3, having aninput/output interface for reading and writing analog voltage levels.

FIG. 7 is a block diagram of one embodiment of an analog I/O data pathin accordance with the memory device of FIG. 6.

FIG. 8 is a block diagram of one embodiment of a data cache circuit inaccordance with the memory device of FIG. 6.

FIG. 9 is a flowchart of one embodiment of a method for programming thememory device of FIG. 6 having the analog data paths.

DETAILED DESCRIPTION

In the following detailed description of the present embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the embodiments may be practiced. These embodiments are describedin sufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that process, electrical or mechanical changes may be madewithout departing from the scope of the present disclosure. Thefollowing detailed description is, therefore, not to be taken in alimiting sense.

Traditional solid-state memory devices pass data in the form of binarysignals. Typically, a ground potential represents a first logic level ofa bit of data, e.g., a ‘0’ data value, while a supply potentialrepresents a second logic level of a bit of data, e.g., a ‘1’ datavalue. A multi-level cell (MLC) may be assigned, for example, fourdifferent threshold voltage (V_(t)) ranges of 200 mV for each range,with each range corresponding to a distinct data state, therebyrepresenting four data values or bit patterns. Typically, a dead spaceor margin of 0.2V to 0.4V is between each range to keep the V_(t)distributions from overlapping. If the V_(t) of the cell is within thefirst range, the cell may be deemed to store a logical 11 state and istypically considered the erased state of the cell. If the V_(t) iswithin the second range, the cell may be deemed to store a logical 10state. If the V_(t) is within the third range, the cell may be deemed tostore a logical 00 state. And if the V_(t) is within the fourth range,the cell may be deemed to store a logical 01 state.

When programming a traditional MLC device as described above, cells aregenerally first erased, as a block, to correspond to the erased state.Following erasure of a block of cells, the least-significant bit (LSB)of each cell is first programmed, if necessary. For example, if the LSBis a 1, then no programming is necessary, but if the LSB is a 0, thenthe V_(t) of the target memory cell is moved from the V_(t) rangecorresponding to the 11 logic state to the V_(t) range corresponding tothe 10 logic state. Following programming of the LSBs, themost-significant bit (MSB) of each cell is programmed in a similarmanner, shifting the V_(t) where necessary. When reading an MLC of atraditional memory device, one or more read operations determinegenerally into which of the ranges the V_(t) of the cell voltage falls.For example, a first read operation may determine whether the V_(t) ofthe target memory cell is indicative of the MSB being a 1 or a 0 while asecond read operation may determine whether the V_(t) of the targetmemory cell in indicative of the LSB being a 1 or a 0. In each case,however, a single bit is returned from a read operation of a targetmemory cell, regardless of how many bits are stored on each cell. Thisproblem of multiple program and read operations becomes increasinglytroublesome as more bits are stored on each MLC. Because each suchprogram or read operation is a binary operation, i.e., each programs orreturns a single bit of information per cell, storing more bits on eachMLC leads to longer operation times.

The memory devices of an illustrative embodiment store data as V_(t)ranges on the memory cells. In contrast to traditional memory devices,however, program and read operations are capable of utilizing datasignals not as discrete bits of MLC data values, but as fullrepresentations of MLC data values, such as their complete bit patterns.For example, in a two-bit MLC device, instead of programming a cell'sLSB and subsequently programming that cell's MSB, a target thresholdvoltage may be programmed representing the bit pattern of those twobits. That is, a series of program and verify operations would beapplied to a memory cell until that memory cell obtained its targetthreshold voltage rather than programming to a first threshold voltagefor a first bit, shifting to a second threshold voltage for a secondbit, etc. Similarly, instead of utilizing multiple read operations todetermine each bit stored on a cell, the threshold voltage of the cellmay be determined and passed as a single signal representing thecomplete data value or bit pattern of the cell. The memory devices ofthe various embodiments do not merely look to whether a memory cell hasa threshold voltage above or below some nominal threshold voltage as isdone in traditional memory devices. Instead, a voltage signal isgenerated that is representative of the actual threshold voltage of thatmemory cell across the continuum of possible threshold voltages. Anadvantage of this approach becomes more significant as the bits per cellcount is increased. For example, if the memory cell were to store eightbits of information, a single read operation would return a singleanalog data signal representative of eight bits of information.

FIG. 1 is a simplified block diagram of a memory device 101 according toan embodiment of the disclosure. Memory device 101 includes an array ofmemory cells 104 arranged in rows and columns. Although the variousembodiments will be described primarily with reference to NAND memoryarrays, the various embodiments are not limited to a specificarchitecture of the memory array 104. Some examples of other arrayarchitectures suitable for the present embodiments include NOR arrays,AND arrays, and virtual ground arrays. In general, however, theembodiments described herein are adaptable to any array architecturepermitting generation of a data signal indicative of the thresholdvoltage of each memory cell.

A row decode circuitry 108 and a column decode circuitry 110 areprovided to decode address signals provided to the memory device 101.Address signals are received and decoded to access memory array 104.Memory device 101 also includes input/output (I/O) control circuitry 112to manage input of commands, addresses and data to the memory device 101as well as output of data and status information from the memory device101. An address register 114 is coupled between I/O control circuitry112 and row decode circuitry 108 and column decode circuitry 110 tolatch the address signals prior to decoding. A command register 124 iscoupled between I/O control circuitry 112 and control logic 116 to latchincoming commands. Control logic 116 controls access to the memory array104 in response to the commands and generates status information for theexternal processor 130. The control logic 116 is coupled to row decodecircuitry 108 and column decode circuitry 110 to control the row decodecircuitry 108 and column decode circuitry 110 in response to theaddresses.

Control logic 116 is also coupled to a sample and hold circuitry 118.The sample and hold circuitry 118 latches data, either incoming oroutgoing, in the form of analog voltage levels. For example, the sampleand hold circuitry could contain capacitors or other analog storagedevices for sampling either an incoming voltage signal representing datato be written to a memory cell or an outgoing voltage signal indicativeof the threshold voltage sensed from a memory cell. The sample and holdcircuitry 118 may further provide for amplification and/or buffering ofthe sampled voltage to provide a stronger data signal to an externaldevice.

The handling of analog voltage signals may take an approach similar toan approach well known in the area of CMOS imager technology, wherecharge levels generated at pixels of the imager in response to incidentillumination are stored on capacitors. These charge levels are thenconverted to voltage signals using a differential amplifier with areference capacitor as a second input to the differential amplifier. Theoutput of the differential amplifier is then passed to analog-to-digitalconversion (ADC) devices to obtain a digital value representative of anintensity of the illumination. In the present embodiments, a charge maybe stored on a capacitor in response to subjecting it to a voltage levelindicative of an actual or target threshold voltage of a memory cell forreading or programming, respectively, the memory cell. This charge couldthen be converted to an analog voltage using a differential amplifierhaving a grounded input or other reference signal as a second input. Theoutput of the differential amplifier could then be passed to the I/Ocontrol circuitry 112 for output from the memory device, in the case ofa read operation, or used for comparison during one or more verifyoperations in programming the memory device. It is noted that the I/Ocontrol circuitry 112 could optionally include analog-to-digitalconversion functionality and digital-to-analog conversion (DAC)functionality to convert read data from an analog signal to a digitalbit pattern and to convert write data from a digital bit pattern to ananalog signal such that the memory device 101 could be adapted forcommunication with either an analog or digital data interface.

During a write operation, target memory cells of the memory array 104are programmed until voltages indicative of their V_(t) levels match thelevels held in the sample and hold circuitry 118. This can beaccomplished, as one example, using differential sensing devices tocompare the held voltage level to a threshold voltage of the targetmemory cell. Much like traditional memory programming, programmingpulses could be applied to a target memory cell to increase itsthreshold voltage until reaching or exceeding the desired value. In aread operation, the V_(t) levels of the target memory cells are passedto the sample and hold circuitry 118 for transfer to an externalprocessor (not shown in FIG. 1) either directly as analog signals or asdigitized representations of the analog signals depending upon whetherADC/DAC functionality is provided external to, or within, the memorydevice.

Threshold voltages of cells may be determined in a variety of manners.For example, a word line voltage could be sampled at the point when thetarget memory cell becomes activated. Alternatively, a boosted voltagecould be applied to a first source/drain side of a target memory cell,and the threshold voltage could be taken as a difference between itscontrol gate voltage and the voltage at its other source/drain side. Bycoupling the voltage to a capacitor, charge would be shared with thecapacitor to store the sampled voltage. Note that the sampled voltageneed not be equal to the threshold voltage, but merely indicative ofthat voltage. For example, in the case of applying a boosted voltage toa first source/drain side of the memory cell and a known voltage to itscontrol gate, the voltage developed at the second source/drain side ofthe memory cell could be taken as the data signal as the developedvoltage is indicative of the threshold voltage of the memory cell.

Sample and hold circuitry 118 may include caching, i.e., multiplestorage locations for each data value, such that the memory device 101may be reading a next data value while passing a first data value to theexternal processor, or receiving a next data value while writing a firstdata value to the memory array 104. A status register 122 is coupledbetween I/O control circuitry 112 and control logic 116 to latch thestatus information for output to the external processor.

Memory device 101 receives control signals at control logic 116 over acontrol link 132. The control signals may include a chip enable CE#, acommand latch enable CLE, an address latch enable ALE, and a writeenable WE#. Memory device 101 may receive commands (in the form ofcommand signals), addresses (in the form of address signals), and data(in the form of data signals) from an external processor over amultiplexed input/output (I/O) bus 134 and output data to the externalprocessor over I/O bus 134.

In a specific example, commands are received over input/output (I/O)pins [7:0] of I/O bus 134 at I/O control circuitry 112 and are writteninto command register 124. The addresses are received over input/output(I/O) pins [7:0] of bus 134 at I/O control circuitry 112 and are writteninto address register 114. The data may be received over input/output(I/O) pins [7:0] for a device capable of receiving eight parallelsignals, or input/output (I/O) pins [15:0] for a device capable ofreceiving sixteen parallel signals, at I/O control circuitry 112 and aretransferred to sample and hold circuitry 118. Data also may be outputover input/output (I/O) pins [7:0] for a device capable of transmittingeight parallel signals or input/output (I/O) pins [15:0] for a devicecapable of transmitting sixteen parallel signals. It will be appreciatedby those skilled in the art that additional circuitry and signals can beprovided, and that the memory device of FIG. 1 has been simplified tohelp focus on the embodiments of the disclosure. Additionally, while thememory device of FIG. 1 has been described in accordance with popularconventions for receipt and output of the various signals, it is notedthat the various embodiments are not limited by the specific signals andI/O configurations described unless expressly noted herein. For example,command and address signals could be received at inputs separate fromthose receiving the data signals, or data signals could be transmittedserially over a single I/O line of I/O bus 134. Because the data signalsrepresent bit patterns instead of individual bits, serial communicationof an 8-bit data signal could be as efficient as parallel communicationof eight signals representing individual bits.

FIG. 2 is a schematic of a portion of an example NAND memory array 200as might be found in the memory array 104 of FIG. 1. As shown in FIG. 2,the memory array 200 includes word lines 202 ₁ to 202 _(N) andintersecting bit lines 204 ₁ to 204 _(M). For ease of addressing in thedigital environment, the number of word lines 202 and the number of bitlines 204 are generally each some power of two.

Memory array 200 includes NAND strings 206 ₁ to 206 _(m). Each NANDstring includes transistors 208 ₁ to 208 _(N), each located at anintersection of a word line 202 and a bit line 204. The transistors 208,depicted as floating-gate transistors in FIG. 2, represent non volatilememory cells for storage of data. The floating-gate transistors 208 ofeach NAND string 206 are connected in series source to drain between oneor more source select gates 210, e.g., a field-effect transistor (FET),and one or more drain select gates 212, e.g., an FET. Each source selectgate 210 is located at an intersection of a local bit line 204 and asource select line 214, while each drain select gate 212 is located atan intersection of a local bit line 204 and a drain select line 215.

A source of each source select gate 210 is connected to a common sourceline 216. The drain of each source select gate 210 is connected to thesource of the first floating-gate transistor 208 of the correspondingNAND string 206. For example, the drain of source select gate 210 ₁ isconnected to the source of floating-gate transistor 208 ₁ of thecorresponding NAND string 206 ₁. A control gate of each source selectgate 210 is connected to source select line 214. If multiple sourceselect gates 210 are utilized for a given NAND string 206, they would becoupled in series between the common source line 216 and the firstfloating-gate transistor 208 of that NAND string 206.

The drain of each drain select gate 212 is connected to a local bit line204 for the corresponding NAND string at a drain contact. For example,the drain of drain select gate 212 ₁ is connected to the local bit line204 ₁ for the corresponding NAND string 206 ₁ at a drain contact. Thesource of each drain select gate 212 is connected to the drain of thelast floating-gate transistor 208 of the corresponding NAND string 206.For example, the source of drain select gate 212 ₁ is connected to thedrain of floating-gate transistor 208 _(N) of the corresponding NANDstring 206 ₁. If multiple drain select gates 212 are utilized for agiven NAND string 206, they would be coupled in series between thecorresponding bit line 204 and the last floating-gate transistor 208_(N) of that NAND string 206.

Typical construction of floating-gate transistors 208 includes a source230 and a drain 232, a floating gate 234, and a control gate 236, asshown in FIG. 2. Floating-gate transistors 208 have their control gates236 coupled to a word line 202. A column of the floating-gatetransistors 208 are those NAND strings 206 coupled to a given local bitline 204. A row of the floating-gate transistors 208 are thosetransistors commonly coupled to a given word line 202. Other forms oftransistors 208 may also be utilized with embodiments of the disclosure,such as NROM, magnetic or ferroelectric transistors and othertransistors capable of being programmed to assume one of two or morethreshold voltage ranges.

Memory devices of the various embodiments may be advantageously used inbulk storage devices. For various embodiments, these bulk storagedevices may take on the same form factor and communication bus interfaceof traditional HDDs, thus allowing them to replace such drives in avariety of applications. Some common form factors for HDDs include the3.5″, 2.5″ and PCMCIA (Personal Computer Memory Card InternationalAssociation) form factors commonly used with current personal computersand larger digital media recorders, as well as 1.8″ and 1″ form factorscommonly used in smaller personal appliances, such as mobile telephones,personal digital assistants (PDAs) and digital media players. Somecommon bus interfaces include universal serial bus (USB), AT attachmentinterface (ATA) [also known as integrated drive electronics or IDE],serial ATA (SATA), small computer systems interface (SCSI) and theInstitute of Electrical and Electronics Engineers (IEEE) 1394 standard.While a variety of form factors and communication interfaces werelisted, the embodiments are not limited to a specific form factor orcommunication standard. Furthermore, the embodiments need not conform toa HDD form factor or communication interface. FIG. 3 is a blockschematic of a solid state bulk storage device 300 in accordance withone embodiment of the present disclosure.

The bulk storage device 300 includes a memory device 301 in accordancewith an embodiment of the disclosure, a read/write channel 305 and acontroller 310. The read/write channel 305 provides foranalog-to-digital conversion of data signals received from the memorydevice 301 as well as digital-to-analog conversion of data signalsreceived from the controller 310. The controller 310 provides forcommunication between the bulk storage device 300 and an externalprocessor (not shown in FIG. 3) through bus interface 315. It is notedthat the read/write channel 305 could service one or more additionalmemory devices, as depicted by memory device 301′ in dashed lines.Selection of a single memory device 301 for communication can be handledthrough a multi-bit chip enable signal or other multiplexing scheme.

The memory device 301 is coupled to a read/write channel 305 through ananalog interface 320 and a digital interface 325. The analog interface320 provides for the passage of analog data signals between the memorydevice 301 and the read/write channel 305 while the digital interface325 provides for the passage of control signals, command signals andaddress signals from the read/write channel 305 to the memory device301. The digital interface 325 may further provide for the passage ofstatus signals from the memory device 301 to the read/write channel 305.The analog interface 320 and the digital interface 325 may share signallines as noted with respect to the memory device 101 of FIG. 1. Althoughthe embodiment of FIG. 3 depicts a dual analog/digital interface to thememory device, functionality of the read/write channel 305 couldoptionally be incorporated into the memory device 301 as discussed withrespect to FIG. 1 such that the memory device 301 communicates directlywith the controller 310 using only a digital interface for passage ofcontrol signals, command signals, status signals, address signals anddata signals.

The read/write channel 305 is coupled to the controller 310 through oneor more interfaces, such as a data interface 330 and a control interface335. The data interface 330 provides for the passage of digital datasignals between the read/write channel 305 and the controller 310. Thecontrol interface 335 provides for the passage of control signals,command signals and address signals from the controller 310 to theread/write channel 305. The control interface 335 may further providefor the passage of status signals from the read/write channel 305 to thecontroller 310. Status and command/control signals may also be passeddirectly between the controller 310 and the memory device 301 asdepicted by the dashed line connecting the control interface 335 to thedigital interface 325.

Although depicted as two distinct devices in FIG. 3, the functionalityof the read/write channel 305 and the controller 310 could alternativelybe performed by a single integrated circuit device. And whilemaintaining the memory device 301 as a separate device would providemore flexibility in adapting the embodiments to different form factorsand communication interfaces, because it is also an integrated circuitdevice, the entire bulk storage device 300 could be fabricated as asingle integrated circuit device.

The read/write channel 305 is a signal processor adapted to at leastprovide for conversion of a digital data stream to an analog data streamand vice versa. A digital data stream provides data signals in the formof binary voltage levels, i.e., a first voltage level indicative of abit having a first binary data value, e.g., 0, and a second voltagelevel indicative of a bit having a second binary data value, e.g., 1. Ananalog data stream provides data signals in the form of analog voltageshaving more than two levels, with different voltage levels or rangescorresponding to different bit patterns of two or more bits. Forexample, in a system adapted to store two bits per memory cell, a firstvoltage level or range of voltage levels of an analog data stream couldcorrespond to a bit pattern of 11, a second voltage level or range ofvoltage levels of an analog data stream could correspond to a bitpattern of 10, a third voltage level or range of voltage levels of ananalog data stream could correspond to a bit pattern of 00 and a fourthvoltage level or range of voltage levels of an analog data stream couldcorrespond to a bit pattern of 01. Thus, one analog data signal inaccordance with the various embodiments would be converted to two ormore digital data signals, and vice versa.

In practice, control and command signals are received at the businterface 315 for access of the memory device 301 through the controller310. Addresses and data values may also be received at the bus interface315 depending upon what type of access is desired, e.g., write, read,format, etc. In a shared bus system, the bus interface 315 would becoupled to a bus along with a variety of other devices. To directcommunications to a specific device, an identification value may beplaced on the bus indicating which device on the bus is to act upon asubsequent command. If the identification value matches the value takenon by the bulk storage device 300, the controller 310 would then acceptthe subsequent command at the bus interface 315. If the identificationvalue did not match, the controller 310 would ignore the subsequentcommunication. Similarly, to avoid collisions on the bus, the variousdevices on a shared bus may instruct other devices to cease outboundcommunication while they individually take control of the bus. Protocolsfor bus sharing and collision avoidance are well known and will not bedetailed herein. The controller 310 then passes the command, address anddata signals on to the read/write channel 305 for processing. Note thatthe command, address and data signals passed from the controller 310 tothe read/write channel 305 need not be the same signals received at thebus interface 315. For example, the communication standard for the businterface 315 may differ from the communication standard of theread/write channel 305 or the memory device 301. In this situation, thecontroller 310 may translate the commands and/or addressing scheme priorto accessing the memory device 301. In addition, the controller 310 mayprovide for load leveling within the one or more memory devices 301,such that physical addresses of the memory devices 301 may change overtime for a given logical address. Thus, the controller 310 would map thelogical address from the external device to a physical address of atarget memory device 301.

For write requests, in addition to the command and address signals, thecontroller 310 would pass digital data signals to the read/write channel305. For example, for a 16-bit data word, the controller 310 would pass16 individual signals having a first or second binary logic level. Theread/write channel 305 would then convert the digital data signals to ananalog data signal representative of the bit pattern of the digital datasignals. To continue with the foregoing example, the read/write channel305 would use a digital-to-analog conversion to convert the 16individual digital data signals to a single analog signal having apotential level indicative of the desired 16-bit data pattern. For oneembodiment, the analog data signal representative of the bit pattern ofthe digital data signals is indicative of a desired threshold voltage ofthe target memory cell. However, in programming of a one-transistormemory cells, it is often the case that programming of neighboringmemory cells will increase the threshold voltage of previouslyprogrammed memory cells. Thus, for another embodiment, the read/writechannel 305 can take into account these types of expected changes in thethreshold voltage, and adjust the analog data signal to be indicative ofa threshold voltage lower than the final desired threshold voltage.After conversion of the digital data signals from the controller 310,the read/write channel 305 would then pass the write command and addresssignals to the memory device 301 along with the analog data signals foruse in programming the individual memory cells. Programming can occur ona cell-by-cell basis, but is generally performed for a page of data peroperation. For a typical memory array architecture, a page of dataincludes every other memory cell coupled to a word line.

For read requests, the controller would pass command and address signalsto the read/write channel 305. The read/write channel 305 would pass theread command and address signals to the memory device 301. In response,after performing the read operation, the memory device 301 would returnthe analog data signals indicative of the threshold voltages of thememory cells defined by the address signals and the read command. Thememory device 301 may transfer its analog data signals in parallel orserial fashion.

The analog data signals may also be transferred not as discrete voltagepulses, but as a substantially continuous stream of analog signals. Inthis situation, the read/write channel 305 may employ signal processingsimilar to that used in HDD accessing called PRML or partial response,maximum likelihood. In PRML processing of a traditional HDD, the readhead of the HDD outputs a stream of analog signals representative offlux reversals encountered during a read operation of the HDD platter.Rather than attempting to capture the true peaks and valleys of thisanalog signal generated in response to flux reversals encountered by theread head, the signal is periodically sampled to create a digitalrepresentation of the signal pattern. This digital representation canthen be analyzed to determine the likely pattern of flux reversalsresponsible for generation of the analog signal pattern. This same typeof processing can be utilized with embodiments of the presentdisclosure. By sampling the analog signal from the memory device 301,PRML processing can be employed to determine the likely pattern ofthreshold voltages responsible for generation of the analog signal.

FIG. 4 is a depiction of a wave form showing conceptually a data signal450 as might be received from the memory device 301 by the read/writechannel 305 in accordance with an embodiment of the disclosure. The datasignal 450 could be periodically sampled and a digital representation ofthe data signal 450 can be created from the amplitudes of the sampledvoltage levels. For one embodiment, the sampling could be synchronizedto the data output such that sampling occurs during the steady-stateportions of the data signal 450. Such an embodiment is depicted by thesampling as indicated by the dashed lines at times t1, t2, t3 and t4.However, if synchronized sampling becomes misaligned, values of the datasamples may be significantly different than the steady-state values. Inan alternate embodiment, sampling rates could be increased to allowdetermination of where steady-state values likely occurred, such as byobserving slope changes indicated by the data samples. Such anembodiment is depicted by the sampling as indicated by the dashed linesat times t5, t6, t7 and t8, where a slope between data samples at timest6 and t7 may indicate a steady-state condition. In such an embodiment,a trade-off is made between sampling rate and accuracy of therepresentation. Higher sampling rates lead to more accuraterepresentations, but also increase processing time. Regardless ofwhether sampling is synchronized to the data output or more frequentsampling is used, the digital representation can then be used to predictwhat incoming voltage levels were likely responsible for generating theanalog signal pattern. In turn, the likely data values of the individualmemory cells being read can be predicted from this expected pattern ofincoming voltage levels.

Recognizing that errors will occur in the reading of data values fromthe memory device 301, the read/write channel 305 may include errorcorrection. Error correction is commonly used in memory devices, as wellas HDDs, to recover from expected errors. Typically, a memory devicewill store user data in a first set of locations and error correctioncode (ECC) in a second set of locations. During a read operation, boththe user data and the ECC are read in response to a read request of theuser data. Using known algorithms, the user data returned from the readoperation is compared to the ECC. If the errors are within the limits ofthe ECC, the errors will be corrected.

FIG. 5 is a block schematic of an electronic system in accordance withan embodiment of the disclosure. Example electronic systems may includepersonal computers, PDAs, digital cameras, digital media players,digital recorders, electronic games, appliances, vehicles, wirelessdevices, mobile telephones and the like.

The electronic system includes a host processor 500 that may includecache memory 502 to increase the efficiency of the processor 500. Theprocessor 500 is coupled to a communication bus 504. A variety of otherdevices may be coupled to the communication bus 504 under control of theprocessor 500. For example, the electronic system may include randomaccess memory (RAM) 506; one or more input devices 508 such askeyboards, touch pads, pointing devices, etc.; an audio controller 510;a video controller 512; and one or more bulk storage devices 514. Atleast one bulk storage device 514 includes a digital bus interface 515for communication with the bus 504, one or more memory devices inaccordance with an embodiment of the disclosure having an analoginterface for transfer of data signals representative of data patternsof two or more bits of data, and a signal processor adapted to performdigital-to-analog conversion of digital data signals received from thebus interface 515 and analog-to-digital conversion of analog datasignals received from its memory device(s).

FIG. 6 illustrates a block diagram of one embodiment of a memory device600, of the bulk storage system of FIG. 3, having an analog input/outputdata interface for reading and writing analog signals. The block diagramof FIG. 6 illustrates only a simplified memory device that highlightsthe elements relevant to the analog I/O data interface of the presentdisclosure. Other elements of the memory device 600 are shown anddiscussed in above-described embodiments or are known by those skilledin the art.

The memory device 600 comprises a memory array 601 that has non-volatilememory cells organized in rows and columns. The rows are coupled to wordlines and the columns are coupled to bit lines. The array format can befabricated as a NAND architecture, a NOR architecture, or some othertype of architecture. The non-volatile memory cells, in one embodiment,are floating gate memory cells.

The memory array 601 is coupled to a plurality of analog data paths 602.In one embodiment, there is one data path for each bit line of thememory array 601. Each analog data path 602 coupled to a bit line isshared with all of the memory cells on that particular bit line. Theselection of a particular word line with a verify voltage causes thatword line to be coupled to its respective analog data path.

The analog data paths 602 act as both a data cache that stores the dataand an input path to access the memory cells in the array 601. The datapaths 602 are located between the analog I/O pads 610 of the memorydevice 600 and the memory array 601. The data paths 602 are coupled tothe 8 or 16 analog I/O pads 610 by an 8 or 16 bit wide bus. Alternateembodiments can use other bus widths. One embodiment of an analog datapath 605 is illustrated in FIG. 7.

The analog I/O data path 605 of FIG. 7 comprises the I/O pad 701 thatincludes a unity gain amplifier 703. The amplifier 703 provides anamplification factor of one to improve the signal strength of the inputanalog voltage. In one embodiment, the amplifier block 703 isbidirectional to enable voltages from the memory array to be output onthe I/O pads 701.

FIG. 8 illustrates an analog data cache circuit that is coupled to theanalog I/O path 605 of FIG. 7. In one embodiment, the data cache circuitis considered part of the analog data path 605 of FIG. 6.

The analog data cache circuit comprises a read circuit 800, a verifycircuit 801, and a reference circuit 802. The circuit of FIG. 8 is forpurposes of illustration only as the data cache function can beaccomplished in many different ways.

The read circuit 800 comprises a voltage storage device 806 that makesup a sample and hold portion of the circuit. The illustrated embodimentuses a capacitor 806 to store the voltage. Alternate embodiments may useanother type of capacitive element or some other means of voltagestorage. The capacitor 806 is connected to the selected word line rampvoltage through a switch 804. The switch is controlled by the controlsignal from the sense amplifier circuit. In operation, the selected wordline ramp voltage increases until it reaches the V_(t) that turns on theselected memory cell. During the ramped voltage, the switch is normallyclosed so that the voltage being stored in the capacitor 806 alsoincreases with the input voltage. When the ramp voltage reaches theV_(t) of the selected memory cell, current begins to flow in the bitline. The sense amplifier detects the current and generates the controlsignal that opens the switch 804. The open switch 804 causes the V_(t)level that initiated the current flow to be stored in the capacitor 806.This is the threshold voltage to which the selected memory cell iscurrently programmed.

The stored threshold voltage is output through an NMOS transistor 805that is connected to a current source 807 through the source connectionof the transistor 805. The drain connection of the transistor 805 isconnected to the supply voltage V_(CC).

The NMOS transistor 805 is connected in a source follower configurationto drive the stored threshold voltage, through an output switch 808, tothe I/O node (i.e., I/O line) of the memory device. The output switch808 is normally open to disconnect the read circuit 800 from the I/Oline. During this time, the I/O switch 820 is closed in order todischarge the I/O line to ground so that any voltage put on the linestarts at 0V. After the V_(t) of the selected cell is stored in thecapacitor 806, the output switch 808 is closed to connect the NMOStransistor 805 to the I/O line and the I/O switch 820 is opened. Acurrent source 821 on the I/O line increases the drive current of theline.

The output of the read circuit 800 will not be the same as the V_(t)stored in the capacitor 804. Since the V_(t) is applied to the gate ofthe NMOS transistor 805, the source of the transistor 805 rises to1.30V−V_(t) where the 1.30V is the gate-to-source voltage drop of thetransistor 805. Thus, if V_(t) is 1.0V, then the read circuit willoutput 0.30V as the read V_(t).

One embodiment, as illustrated in FIG. 8, uses a reference circuit 802.This reference circuit 802 is substantially similar to the read circuit800 in that it comprises a switch controlled by the sense amplifiercontrol signal, a storage capacitor 826, an NMOS transistor 825configured in a source follower configuration with a current source 827on the source connection, and an output switch 828 that is open untilthe V_(t) is stored in the capacitor 826 and the input switch 824 isopened.

The reference circuit 802 operates by the memory controller sending acommand to a voltage source to store the target V_(t) of the selectedmemory cell in the capacitor 826 of the reference circuit 802. The inputswitch 824 is then opened by the controller to contain the target V_(t)in the capacitor 826. The reference circuit 802 can then drive thisvalue out, through the output switch 828, to the I/O line. As discussedpreviously, the I/O line is first discharged by the discharge switch 820so that the output voltage starts at 0V. Even though the same voltagedrop exists across the transistor 825 as in the read circuit 800, thememory controller now knows the actual V_(t) that was stored in thereference circuit 802. When the reference circuit 802 output is readfrom the I/O line by the memory controller, the controller knows theV_(t) value that corresponds to the voltage that was read from the I/Oline. Thus, when the controller reads this same voltage during the timethat the read circuit 800 is driving its voltage onto the I/O line, itknows the V_(t) that was stored in the read circuit capacitor 806.

The read circuit 800 output and the reference circuit 802 output can bealternately connected to the I/O line by the memory controller duringseparate read cycles. The controller can alternate the closing of therespective output switches 808, 828 of each circuit 800, 802 in order toput the desired output onto the I/O line. The I/O line is coupled to theunity gain amplifier 703 of FIG. 7.

The reference circuit has the added benefit of correcting the readcircuit V_(t) for temperature variations. Since the reference circuitoutput voltage will vary in a similar manner as the read circuit outputand the stored V_(t) value in the reference circuit is known, the memorycontroller can determine the actual V_(t) stored in the read circuit bya translation table stored in memory.

The verify circuit 801 includes a comparator function 815 that, in oneembodiment, comprises an operational amplifier configured as acomparator 815. The comparator circuit 815 compares the voltage from theread circuit 800 output with the voltage from a verify circuit sourcefollower transistor 812 output. The comparator circuit 815 then outputsan INHIBIT signal when the two signals are substantially equal. TheINHIBIT signal is used to inhibit programming of the memory cell thathas reached its threshold voltage. The verify circuit source followertransistor 812 is coupled to a current source 813 on the sourceconnection.

For operation of the circuit, the analog voltage to be programmed intothe cell is loaded into the sample/hold circuit. This is accomplished byclosing the switch 810 so that the incoming data is sampled by capacitor811. The switch 810 is then opened and the capacitor 811 now holds thetarget data.

The selected cell is then programmed as described subsequently. Eachprogramming pulse applied to the selected cell moves V_(t) a certainthreshold voltage distance. A read and verify operation is performedbetween each programming pulse to determine if the V_(t) has reached thetarget voltage.

The verification operation comprises storing the target V_(t) in a datastorage device, such as a capacitor 811, of the verify circuit 801. Thiscan be accomplished during the verify operation or at the same time aswhen the capacitor 826 in the reference circuit 802 was programmed withthe target V_(t). After the verify capacitor 811 has been programmed,the input switch 810 is opened to store the voltage on the capacitor811. A read operation is then performed.

As discussed previously, the read operation comprises a representationof a ramp voltage being applied to the input of the read circuit 800until V_(t) is reached and stored in the capacitor 806. The output ofthe source follower transistor 805 is then applied to the input of thecomparator circuit 815. If the cell V_(t) is less than the target V_(t),the INHIBIT signal indicates (e.g., a logical low signal) that the cellneeds an additional program pulse. The above-described programmingsequence is then repeated. If the cell V_(t) is substantially equal toor higher than the target V_(t), the INHIBIT signal indicates (e.g., alogical high signal) that the cell does not need any further programmingpulses and the cell is put into the “inhibit” state.

The “inhibit” state is indicated when the output of the source followertransistor 805 of the read circuit is at least equal to the output ofthe source follower transistor 812 of the verify circuit 801. At thispoint, the comparator circuit 815 outputs an INHIBIT signal. In oneembodiment, the INHIBIT signal is a logical 1. The INHIBIT signal isused to initiate an inhibit function.

The inhibit function can be accomplished using various methods inresponse to a circuit receiving the INHIBIT signal. For example, the bitline bias can be changed from the program enable voltage of 0V, usedduring a programming operation, to V_(CC) that inhibits programming ofmemory cells coupled to that particular bit line. The bit line voltagecan also be varied between 0V and V_(CC) to slow programming instead ofcompletely inhibiting programming.

The representation of the analog ramp voltage for the above embodimentscan be a conditioned version of a selected word line ramp voltage.Conditioning operations include reducing the voltage range (e.g.,dividing the selected word line ramp voltage by 5), level shifting(e.g., shifting the selected word line ramp voltage where −2V to +3Vchanges to +2V to +3V), and buffering.

One embodiment of an operation of the circuit of FIG. 6 is illustratedin the flowchart of FIG. 9. The method begins when the memory devicereceives an address from which the programming is to start 900. Thecontroller then stores an analog voltage 901 in the analog data paththat is associated with the start address. This analog voltage is thevoltage that is to be written to the memory cell currently associatedwith the analog data path. The associated memory cell is indicated bythe selected memory cell at the intersection of a word line and theassociated bit line.

As discussed previously, the analog voltage to be written to theselected memory cell is representative of a multiple bit pattern to bestored in the selected memory cell. This bit pattern can be two or morebits, each bit pattern being represented by a different thresholdvoltage. Another embodiment stores only a single bit in each memorycell.

The data path currently associated with the present memory cell addressis then checked to determine if it is the final data path forprogramming 902. The final data path may be the last one as indicated bythe memory controller in a length command (as measured from the startaddress), in a final address command, the last data path for a memorypage or block being programmed, or some other way of determining thefinal analog data path for programming.

If the data path being programmed is not the final data path 902, theprogramming is clocked or incremented to the next data path 920 in thepage or block. The next data path is then programmed with the analogvoltage and the process repeated until the final data path is reached902.

Once all of the desired analog data paths have been loaded with theappropriate analog voltage (i.e., data) to be programmed into theirrespective memory cells, the voltages are then transferred to therespective memory cells. This is accomplished through a memory cellprogramming/verification process.

A target voltage that is representative of the desired analog voltage(i.e., target data) that is to be programmed into the selected memorycell is stored in the verify circuit portion of the sample/hold circuit903. In an alternate embodiment, the reference circuit is alsoprogrammed with this data. An initial programming pulse is thengenerated to bias the word line that is connected to the control gate ofthe selected memory cell 904.

During a typical programming operation, the selected cell is biased by aseries of incrementally increasing programming pulses. A memory celltypically starts the programming operation in an erased state with anegative threshold voltage. Each programming pulse increases thethreshold voltage, V_(t), of the memory cell a certain voltage dependingon the programming voltage pulse level.

The verify operation as described previously is then performed on theselected memory cell 905 to determine if it is programmed to the targetthreshold voltage 911. The verify operation determines if the selectedcell threshold voltage is greater than or equal to the stored targetvoltage.

As described previously, the verify operation includes biasing the wordline with a ramp voltage until the memory cell starts to conduct andproduce a current on the bit line. Once the current sensing circuitdetects the bit line current, it generates a control signal indicatingto the sample/hold circuitry to store the current ramped read voltage,or an indication of the current ramped read voltage, that caused thecell to turn on. The stored, target analog voltage is compared to thesample and hold voltage from the ramped read voltage in order todetermine if the selected memory cell has been programmed to the targetthreshold voltage 911. In other words, the selected cell is checked todetermine if the target data has been programmed.

If the selected memory cell has been programmed 911, further programmingof the selected cell is inhibited 915. The bit line inhibit can beaccomplished as discussed previously or using some other inhibit method.

If the selected memory cell has not yet reached the target thresholdvoltage 911, the programming voltage is increased 913. Anotherprogramming pulse at the increased programming voltage is then generatedand the process repeated until the threshold voltage of the selectedcell is substantially the same as the stored analog voltage to beprogrammed. The threshold voltage of the selected cell does not have tobe exactly equal to the desired analog voltage in order for the selectedcell to be considered programmed. The cell may be under or overprogrammed by hundredths or thousandths of a volt and still beconsidered programmed.

CONCLUSION

One or more embodiments of the present disclosure provide an analog I/Odata interface with a memory device adapted to store analog voltagesrepresentative of digital bit patterns. One such analog I/O datainterface, comprised of a plurality of analog data paths having storageand comparison capabilities, stores a target voltage for each bit lineand compares a threshold voltage on a respective programmed cell withthe stored target voltage. The data path then inhibits furtherprogramming once the target voltage is reached.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe disclosure will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the disclosure.

1. An analog input/output data interface comprising: an analog interfaceconfigured to couple a memory array to an external circuit; and ananalog data cache, coupled between the analog interface and the memoryarray, for storing an analog signal representative of data, wherein theanalog interface and the analog data cache are configured to accept theanalog signal from the external circuit for storing in the memory array,the analog data cache comprising a comparator circuit coupled to astorage element of the analog data cache and a selected memory cell ofthe memory array, the comparator circuit configured to compare a signalfrom the storage element with a voltage from the selected memory celland generate an inhibit signal that inhibits further programming inresponse to the comparison.
 2. The interface of claim 1 wherein theanalog signal is representative of a digital bit pattern.
 3. Theinterface of claim 1 wherein the analog data cache comprises a voltagestorage device that is configured to store the analog signal.
 4. Theinterface of claim 1 wherein the analog interface comprises a circuitconfigured to produce unity gain.
 5. The interface of claim 4 whereinthe circuit to produce unity gain is bidirectional to enable the memoryarray to output a signal to the external circuit.
 6. The interface ofclaim 5 wherein the signal output by the memory array is an analogsignal representative of a digital bit pattern stored in the memoryarray.
 7. A memory device comprising: an array of memory cells; and ananalog input/output data interface coupled to the memory array, theinterface comprising a plurality of analog data paths, each analog datapath comprising a storage element configured to store an analog signal,input to the memory device, that is to be programmed into a selectedmemory cell of the memory array, the analog input/output data interfacefurther comprising a comparator circuit coupled to the storage elementand the selected memory cell, the comparator circuit configured tocompare a signal from the storage element with a voltage from theselected memory cell and generate an inhibit signal that inhibitsfurther programming in response to the comparison.
 8. The device ofclaim 7 and further including a memory controller that is configured tocontrol programming of the analog signal input to the selected memorycell.
 9. A non-volatile memory device comprising: an array of memorycells coupled to bit lines; and an analog I/O interface coupled to thememory array through the bit lines, the interface comprising a pluralityof analog data paths, each analog data path coupled to a different bitline and comprising: a read circuit configured to store a thresholdvoltage of a selected memory cell and output an indication of thethreshold voltage to an I/O node of the memory device; and a verifycircuit configured to store a target threshold voltage for the selectedmemory cell and comprising a comparison circuit configured to generatean inhibit signal in response to a comparison of the threshold voltagefrom the read circuit and the target threshold voltage.
 10. The deviceof claim 9 wherein the read circuit comprises a sample and hold circuit.11. The device of claim 10 wherein the sample and hold circuit comprisesa switch coupled to a capacitive element and the read circuit furthercomprises a source follower transistor coupled to the capacitive elementand the switch wherein an output of the source follower transistor iscoupled to the comparison circuit and switchably coupled to the I/Onode.
 12. The device of claim 9 wherein each analog data path furthercomprises a reference circuit, coupled to the I/O node, that isconfigured to correct the read circuit for temperature variations. 13.The device of claim 12 wherein the reference circuit comprises: an inputswitch coupled to a voltage source; a capacitive element coupled to theinput switch; and a transistor in a source follower configuration havinga gate connection coupled to the input switch and capacitive element andan output switchably coupled to the I/O node.
 14. The device of claim 13wherein the reference circuit further comprises a current source coupledto a source connection of the transistor.
 15. A method for operating amemory device having a plurality of analog data paths coupled to amemory array, the method comprising: storing an analog signal in ananalog data path, of the plurality of analog data paths, that isassociated with a selected memory cell; programming a target thresholdvoltage into a data cache circuit of the analog data path that isassociated with the selected memory cell; applying a programming pulseto the selected memory cell to program the selected memory cell with theanalog data signal; determining a threshold voltage of the selectedmemory cell; comparing, in the analog data path, the threshold voltageof the selected memory cell with the target threshold voltage; andinhibiting programming when the threshold voltage of the selected memorycell is equal to or greater than the target threshold voltage.
 16. Themethod of claim 15 wherein comparing comprises: a comparison circuit ofa verify circuit of the associated analog data path comparing the targetthreshold voltage to the threshold voltage of the selected memory cell;and the comparison circuit generating an inhibit signal when thethreshold voltage of the memory cell is equal to or greater than thetarget threshold voltage.
 17. The method of claim 15 and furthercomprising generating a reference voltage that temperature compensatesdetermining the threshold voltage of the selected memory cell.
 18. Themethod of claim 15 wherein storing the analog signal further comprises:incrementing through the plurality of analog data paths; and storing ananalog signal in each of the plurality of analog data paths.
 19. Themethod of claim 16 wherein determining a threshold voltage of theselected memory cell comprises: applying a ramped voltage to a controlgate of the selected memory cell until the selected memory cell producesa current on an associated bit line; and causing sample and holdcircuitry of the verify circuit to store an indication of the rampedvoltage that produced the current.